Since pen input devices are very useful for the image input of diagrams and the like and are miniature, they are taking the place of other information input means such as keyboards and mouses. Particularly, by combining a display device with a coordinate detection device in one body, input images are displayed rapidly on the display device, and the handling of the device has been improving.
In order to manufacture a coordinate input device having displaying properties at a low price, a display-integrated coordinate input device is disclosed which carries out display and coordinate detection by using time sharing while applying a display electrode, used for the display of a matrix panel such as a liquid crystal display, and also as an electrode for detecting coordinates (Published Unexamined (Laid-open) Japanese Patent Application No. Hei 2-255911).
A conventional display-integrated coordinate input device is explained below.
The principles of a display-integrated tablet in an electrostatic capacity coupling method is disclosed in Published Unexamined (Laid-open) Japanese Patent Application No. Sho 54-24538 and Published Unexamined (Laid-open) Japanese Patent Application No. Sho 62-180417.
As shown in FIG. 37, a conventional display-integrated coordinate input device includes a matrix panel (for example, a liquid crystal panel) 11 having display elements connected to each intersection of mutually orthogonal row electrodes Y.sub.1 -Y.sub.n and column electrodes X.sub.1 -X.sub.m, an electrode drive circuit consisting of a row electrode drive circuit 12 for driving row electrodes and a column electrode drive circuit 13, a detection electrode 2 for detecting the scanning signals of matrix panel 11, a coordinate detection circuit 3 consisting of a row coordinate detection circuit and a column coordinate detection circuit for detecting row and column coordinates respectively from signals obtained by detection electrode 2, and a control circuit 7 for controlling row electrode drive circuit 12, column electrode drive circuit 13 and coordinate detection circuit 3.
In a display period, a coordinate detection pulse is continuously supplied at one electrode unit from row electrode drive circuit 12 to the row electrodes of matrix panel 11. Along with the coordinate detection pulse supplied to the row electrodes of matrix panel 11, voltage in response to display data is supplied from column electrode drive circuit 13 to the column electrodes of matrix panel 11, providing display.
On the other hand, in a row coordinate detection period, coordinate detection pulse is continuously supplied from the row electrode drive circuit 12 to the row electrodes of matrix panel 11. When the detection electrode 2 is contacted to the predetermined position of matrix panel 11, the row coordinate detection circuit 3 detects the row coordinate at the detection electrode contacting position from a coordinate detection pulse, detected through electrostatic coupling capacity between the detection electrode 2 and the row electrodes. In a column a coordinate detection period, coordinate detection pulse is continuously supplied from the column electrode drive circuit 13 to the column electrodes of the matrix panel 11. When the detection electrode 2 is contacted to a predetermined position of matrix panel 11, column coordinate detection circuit 3 detects the column coordinate at the detection electrode contacting position from a coordinate detection pulse detected through electrostatic coupling capacity between the detection electrode 2 and the column electrodes. Therefore, one matrix panel is used to display images and detect coordinates.
Since obtained row and column coordinate data is immediately displayed as display data, input images such as diagrams and letters look as if they are handwritten on a flat surface.
In addition, the coordinate detection circuit 3 detects a coordinate detection pulse through electrostatic coupling capacity between the detection electrode 2 and the row electrodes, and coordinates are detected from the time when the output becomes maximum (called "peak detection coordinate").
The Positions of coordinates are detected by applying the time when detecting signals become a maximum, so that a positional mistake caused by the height of detection electrode 2 is prevented. This can be understood from a simple model shown in FIG. 38 which detects electrostatic induction signals appearing on the detection electrode 2 by shifting a scanning electrode, connected to power source V.sub.0, at a fixed speed v on a sheet electrode, connected to ground.
Suppose a coordinate at the center of the driving electrode is x.sub.c ; the width of the electrode is 2x.sub.w ; the position of the detection electrode is P(x.sub.p, y.sub.p, h.sub.p); and coupling capacity per unit area of each section of the scanning electrode is C(x, y). Then, output V(t) is expressed as in the following Formula 1. ##EQU1##
The denominator is set as in the following Formula 2. ##EQU2##
The following Formula 3 is provided, when a coordinate at the center of the driving electrode is x.sub.c, scanning speed is v, and time from the beginning of scanning is t. EQU .chi..sub.c =.nu.t-.chi..sub.w Formula 3
Thus, under the conditions that the coupling capacity between detection electrode 2 and the drive electrode stay constant, the following Formula 4 is found when output V(t) is differentiated with time t. ##EQU3##
Under conditions that (1) V(x, y) does not vary in response to x and (2) C(x, y, h) is an even function in regard to x, the differentiated value of output V(t) becomes 0 when the following Formula 5 is satisfied. EQU .chi..sub.c =.chi..sub.p Formula 5
Thus, by utilizing the appearance time of the maximum detection signal, the y coordinate of detection electrode 2 is detected, and the x coordinate of detection electrode 2 can be detected without being dependent on h (height) coordinate. Similarly, the x coordinate of detection electrode 2 is detected, and the y coordinate of detection electrode 2 can be detected without being dependent on the h coordinate.
Methods of determining the appearance time of the maximum value include (a) a method of detecting the zero point by using a differential filter (in this case, the potential change of the maximum value is 0), (b) a method of detecting edges and a mid-point (treatment of an average value) (The appearance time of the peak is the same as the average time of appearance times of leading edge and trailing edge of comparator output.) and (c) a method of detecting peak values by a curve of the second order (by approximating a detection signal waveform around the maximum value, the peak appearance time is measured based on the potential information of three points or more). A method of determining the appearance time of the maximum value is selected from these methods.
However, the following problems are found when a STN-type liquid crystal panel is used as a matrix panel.
(1) For example, the electrode width is 270 .mu.m and wide relative to 300 .mu.m electrode pitch, and 90% of the induction signals from the electrode on the back layer is shut off. Especially when a TFT active matrix panel or the like is used, the effective area is further restricted.
(2) The height of a pen (detection electrode) is limited by applying a protective plate such as an acrylic plate and glass to protect the panel, so that the intensity of detecting signals is weakened. Thus, in order to increase the intensity, it is necessary to widen the width of the coordinate detection pulse (in other words, increasing the number of scanning electrodes at the same time) so as to maintain the detection intensity. However, due to the structural problem of the panel in that an electrode used for detecting coordinates is placed only in a display area, x.sub.w becomes small in the peripheral section when the signal intensity is at maximum, thereby generating a positional difference.
(3) Since the electrode width is quite wide as described in (1) and the cell thickness of the panel is 6-8 .mu.m and thin, the coupling capacity between the electrodes is large. Also, the transmission of signals, transmitting ITO, is delayed because the electrode resistance of ITO is large, thus generating a positional difference.
The positional difference, caused by the structure around leading electrodes which are led to the row and column drive circuits of the matrix panel, is explained below.
FIG. 39 is a perspective view of a section of the matrix panel of a conventional display-integrated coordinate input device using a STN-type liquid crystal panel. As shown in FIG. 39, the matrix panel includes a first glass substrate 310 placed on top, a second glass substrate 320 placed on the bottom, and a liquid layer 35 disposed between the first glass substrate 310 and the second glass substrate 320. On first glass substrate 310, column electrodes 15 (x coordinates) are formed at a first predetermined direction at a first predetermined pitch. Row electrodes 19 (y coordinates) are formed in second glass substrate 320 at a second predetermined direction which is at right angle to the first predetermined direction, at a second predetermined pitch (in general, the same as the first predetermined pitch).
Generally, a section where column electrodes 15 and row electrodes 19 are indirectly laminated to each other in space is a display area, and a section besides the area is a non-display area. At the edges of glass substrate 310, which are at right angles to the first predetermined direction, each tab 34 is applied to the predetermined number of column electrodes 15. As a whole, a plurality of tabs 34 is arranged in the first predetermined direction. Tabs 34 are to connect column electrode drive circuit 13 and column electrodes 15 shown in FIG. 37. The pitch of connecting terminals or the like on tabs 34 is different from the first pitch of column electrodes 15, and is generally narrower than the first pitch. Therefore, at the non-display area on first glass substrate 310, each column electrode 15 is connected to the connecting terminals or the like of tabs 34, so that parallel connecting electrodes 313, having the same pitch as the connecting terminals or the like of tabs 34, and non-parallel (inclined) leading electrodes 312 or the like which connect parallel connection electrodes 15 to column electrodes 311 are formed. Similarly, at the edges of glass substrate 320, which are at right angles to the second predetermined direction, each tab 34 is applied to the predetermined number of row electrodes 19. As a whole, a plurality of tabs 34 is arranged in the second predetermined direction. Tabs 34 connect row electrode drive circuit 12 and row electrodes 19 as shown in FIG. 37. The pitch of connecting terminals or the like on tabs 34 is different from the second pitch of row electrodes 19. Therefore, at the non-display area on second glass substrate 320, each row electrode 19 is connected to the connecting terminals or the like of tabs 34, so that parallel connecting electrodes 323, having the same pitch as the connecting terminals or the like of tabs 34, and non-parallel (inclined) leading electrodes 322 or the like which connect connection electrodes 323 to row electrodes 19 are formed. A triangular dummy electrode 16 is formed around the neighboring section of two tabs 34. Dummy electrodes 16 normally prevent the accumulation of charge, the leakage of light, etc. during manufacturing. The dummy electrodes are connected to a line adjacent to leading electrodes 312 and 322.
The detection of coordinates at the panel edge of the display-integrated coordinate input device using the conventional matrix panel will now be explained.
In FIG. 39, suppose the first predetermined direction of arranged column electrodes 15 is X, and the second predetermined direction of arranged row electrodes 19 is Y. When detection electrode 2 is pointed at the center of matrix panel 11, X and Y coordinates are detected with about 1 dot precision. However, when detection electrode 2 is shifted, for example, in the X direction on line 500 near the edge of the matrix panel 11, a positional difference equivalent to several dots is generated in the X direction due to coordinate detection pulse applied to dummy electrodes 16. There is also a positional difference due to the shape of non-parallel leading electrodes 312. For instance, when detection electrode 2 is shifted in the Y direction on line 600 of matrix panel 11, a positional difference equivalent to several dots is found in the X direction. This phenomenon varies in response to the shape of non-parallel leading electrodes 312. These positional differences are also found with respect to leading electrodes 322.
Therefore, the display-integrated coordinate input device using the conventional matrix panel generates positional differences near the edges of matrix panel 11, which is caused by the coordinate detection pulse applied to dummy electrodes 16 and the shapes of non-parallel leading electrodes 312 and 322. In other words, the device cannot provide preferble coordinate detection precision at the edges of the panel.
The driving voltage of a coordinate detection pulse in the conventional display-integrated coordinate input device is explained below.
FIG. 40 shows one embodiment of a voltage waveform applied to the electrodes of the matrix panel of the conventional display-integrated coordinate input device. In the figure, T.sub.w is a display period and T.sub.d is a detection period; T.sub.w is generally longer than T.sub.d. Also, t.sub.1 is the scanning period of one line in T.sub.w, t.sub.2 is the scanning period of one line in T.sub.d, with t.sub.1 being much longer than t.sub.2. R.sub.1, R.sub.2 and R.sub.m are waveforms applied to row electrodes; S.sub.1, S.sub.2 and S.sub.n are waveforms applied to column electrodes.
In T.sub.w, voltage at the level of V.sub.0 or V.sub.5 is applied to the row electrodes in a selection period, and voltage at the level of V.sub.1 or V.sub.4 is applied in a non-selection period. When the display is ON, voltage at the level of V.sub.0 or V.sub.5 is applied to the column electrodes. Voltage at the level of V.sub.2 or V.sub.3 is applied to the electrodes if the display is OFF.
On the other hand, a coordinate detection pulse is applied continuously to the row and column electrodes in T.sub.d as described above. In order not to leave direct current in T.sub.d, V.sub.0 and V.sub.5 are applied in the selection period and in the non-selection period respectively, even though there are various amplitudes of applied pulse.
FIG. 41 shows an example of conventional liquid crystal drive power circuits. As shown in this figure, a circuit includes resistances r and R for dividing liquid crystal driving voltage for display (V.sub.LCD) to voltage V.sub.0, V.sub.1, V.sub.2, V.sub.3, V.sub.4 and V.sub.5, and operation amplifiers 901, 902, 903 and 904. When the drive conditions of the liquid crystal display vary due to a change in temperature or the like, V.sub.0 -V.sub.5 is changed by varying V.sub.LCD so as to provide preferable display.
In the above-noted conventional driving methods, the voltage level is determined by V.sub.LCD, and the voltage level of a driving pulse applied to the electrodes in a display period is equal to the voltage level of a coordinate detection pulse applied to the electrodes. Therefore, when the drive conditions of the display change, it is necessary to vary driving voltage.
However, if the driving voltage is changed, the amplitude of a coordinate detection pulse in a detection period also changes, thus changing the intensity of signals detected by the detection electrode and varying detection precision.
As described above, conventional display-integrated coordinate input devices have poor detection precision particularly at the edges of the matrix panels, so that they are not suitable for input devices which require detection precision at the edges of panels, such as Windows.